The present invention relates to a method for producing a long ceramic body, and more specifically, to a method for producing a tubular ceramic body used, for example, in a heat transfer tube for a shell-and-tube-type heat exchanger and the like.
Heretofore, long ceramic members for the use in heat transfer tubes for ceramic shell-and-tube-type heat exchangers and the like (hereafter referred to as "long ceramic bodies") have been manufactured. Such long ceramic bodies are normally manufactured by calcining molded bodies as required obtained by molding material powders using various molding methods into desired long shapes, then heating and firing the molded bodies suspended in a sheath so as not to contact them directly with the interior surface of the sheath.
In order to prevent warp in the lower part of the long body due to slight difference in density within the molded body during the firing process, the long body 5 is first fired until sintering is completed at a temperature at which sintering can take place, as FIG. 2(a) shows, then the long body 5 is inverted upside down and fired again at a temperature at which sintering can take place. When the long body 5 is fired under the condition in which it is suspended, warping is inhibited in the upper part of the long body 5 where a strong downward tensile force is applied by its self weight, whereas warping occurs in the lower part of the long body 5 during the firing process because little force that inhibits deformation is applied to the lower part. Therefore, by inverting the lower part with large warping so as to become the upper part, and firing again at a temperature at which plastic deformation takes place, the lower part (the upper part during second firing) warped during first firing is pulled downwards by the self weight of the long body 5, and warping is corrected.
In recent years, however, although increase in the length of heat transfer tubes has been investigated in the development of ceramic shell-and-tube-type heat exchangers, problems arise on difficult assembling the tubes with tube plates by firing as the length of heat transfer tubes increases due to the warp of the tubes which has not caused problems if the length of the tubes is short. Therefore, the development of a method for producing long ceramic bodies with higher accuracy of shapes such as straightness compared with conventional short tubes. Specifically, the achievement of straightness of 0.5 mm or less is desired.
Under such situations, since the conventional methods in which the long bodies are merely inverted upside down can achieve the straightness of up to about 1.0 mm, and the manufacture of long ceramic bodies to meet requirements in heat transfer tubes with increased length has been difficult.
Also in conventional methods, since warp was corrected only by the self weight of long bodies, variation was caused in the correction of warp, and the accuracy in the shape of resulted long ceramic bodies was fluctuated.
The present invention was accomplished in consideration of such situations, and it is an object of the present invention to provide a method for producing a long ceramic body with a high shape accuracy with low fluctuations of shape accuracy, by controlling the deformation of the long ceramic body during the firing process.